1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to radio frequency (radar) antennas for missile seekers and other applications.
2. Description of the Related Art
Radio frequency (RF) antennas are used in many communication, ranging and detection (radar) applications. In missile applications, the RF antenna is implemented as part of a missile seeker. The seeker comprises the antenna along with a transmitter and a receiver. Typically, missile seekers transmit and receive a beam having a single polarization. The polarization of a beam is the orientation of the electric field thereof. Hence, the polarization of a beam may be vertical, horizontal or circular.
Several dual polarization antennas are known in the art. One is a reflector antenna with dual polarization feed. This type of antenna is bulky, exhibits poor efficiency, and poor isolation between the two polarizations. This type of antenna is also very limited in its ability offer low sidelobe radiation performance. Furthermore, this type antenna can generally be used only for an electrically very large aperture (i.e. an aperture having a diameter larger than fifteen wavelengths).
A second approach involves the use of an array of dual polarized patches. This type of antenna offers low cost and low profile, but the bandwidth of each element is typically so narrow that it is very difficult to achieve high performance. The efficiency of this array is also typically poor due to dielectric losses and stripline conductor losses.
A third approach involves the use of a dual polarization rectangular waveguide array consisting of a stack-up of a rectangular waveguide-fed offset longitudinal slot array and a waveguide-fed tilted edge slot array. Unfortunately, this array exhibits poor performance because the offset slot excites an undesirable TM01 odd mode in the parallel plate region formed by the tilted edge slot waveguides. The excited TM01 odd mode causes high sidelobes and RF loss. A further performance limitation results from the coupling between apertures caused by the tilted edge slot containing a cross-polarization component.
A fourth approach involves the use of an arched notch dipole card array erected over a rectangular waveguide fed offset longitudinal slot array. In this approach, the arch is provided to improve the performance of the principal polarization slot array and minimize interactions between the two apertures. Unfortunately, the design of this type of array is very difficult because there is no easy or convenient method to account for the presence of the arched dipole array in the design of the slot array (every slot sees a different unit cell). The requirement to maximize the spacing between the face of the slot array and the arch cards to reduce interaction conflicts with the desired placement of the notch radiators on the quarter-wavelength above this surface for optimal image current formation. This limitation becomes especially severe at higher frequencies of operation.
Finally, a fifth approach involves the use of a common aperture for dual polarization array with a flat plate centered longitudinal shunt slot array and a stripline-fed notch-dipole array. This approach was disclosed and claimed in U.S. Pat. No. 6,166,701 issued Dec. 26, 2000 to Pyong K. Park et al. and entitled DUAL POLARIZATION ANTENNA ARRAY WITH RADIATING SLOTS AND NOTCH DIPOLE ELEMENTS SHARING A COMMON APERTURE the teachings of which are incorporated herein by reference. This approach is very useful for very high frequency (Ka-band or higher) applications and electrically medium to large size arrays. For lower frequency applications such as X-band, and small diameter apertures, such as under seven wavelengths, the dipole card height is greater than a half-inch, which is often more than the available antenna depth. Therefore, it may not be practical to use this approach for lower frequency applications and electrically small to medium size antennas.
Accordingly, inasmuch as current trends in radar communication and antenna system design requirements emphasize the reduction of cost and volume while achieving high performance, a need exists in the art for an antenna design which offers an improved capability.
The need in the art is addressed by the dual-polarization common aperture antenna of the present invention. The inventive antenna includes first and second arrays of radiating slots disposed in a faceplate. The second array is generally orthogonal and therefor cross-polarized relative to the first array. The first array is waveguide fed and the second array is inverted micro-stripline fed.
In the illustrative implementation, the first array and the second array share a common aperture. The common aperture is fully populated and each array uses the aperture in its entirety. The first and second arrays of slots are arranged for four-way symmetry. Each slot in the first array is a horizontally oriented, iris-excited shunt slot fed by a rectangular waveguide and centered on a broad wall thereof. The second array is a standing wave array in which each slot is an air cavity backed slot fed by an inverted micro-stripline offset from a center thereof.